In the field of telecommunications, lasers such as vertical cavity surface emitting lasers (VCSELs) and other opto-electronic devices are commonly used for the transmission of information along optical fibers and the like. VCSELs, in particular are especially desirable in today's optical communication systems because they are efficient, small in size, readily assembled into arrays, and easy to manufacture.
Within optical communication systems utilizing VCSELs or other lasers, it is often desirable to control the parameters of the optical data signal being transmitted. For example, it is often desirable to control average power and amplitude of the signal. If the average power Pave is maintained properly, the laser may be modulated about the average power bias point at a modulation level necessary to achieve desired high and low light output power levels, Phigh and Plow.
An optical modulation amplitude (OMA) and an extinction ratio (ER) of the laser, defined as Phigh-Plow and Phigh/Plow, respectively, is commonly maintained within predetermined limiting values to maintain desired optical signal integrity. The limit values commonly are per specification such as the Synchronous Optical NETwork (SONET) or Gigabit Ethernet specification, or any other specification that the system is designed to meet.
Therefore, in order to obtain reliable and repeatable results in many fiber optic transmission applications, it is desirable to maintain both the average signal power and the OMA (or ER) within predetermined limit values. Unfortunately, laser characteristics change during the operation of the laser. In particular, as a laser such as a VCSEL is used to transmit optical data, the temperature of the operating laser and the environment which contains it, typically tends to increase, which may degrade laser performance. The OMA also changes as the temperature of the operating environment changes, and the change of the OMA with temperature is typically dependent on the particular laser used and the age of the particular laser. For example slope efficiency, a measure of optical output per current used to drive the laser, of the lasers may change due to temperature and age of the lasers. Automatic power control may be used to ameliorate these problems. Automatic power control is also used to account for laser threshold changes.
Similar to many other electronic systems, it is desirable to limit the power used by the laser drivers to drive the lasers. The limiting of the power used by the laser drivers to drive lasers result in reduction to power requirements and also reduces heat dissipation. Due to the reduced heat dissipation, the reduction in power requirements may also result in improvement of the laser performance due to reduced increase in temperature.